[1] IRC logs: freenode / #whatwg / 20120713 ( (2012-07-21 01:41:48 +09:00 版)) <http://krijnhoetmer.nl/irc-logs/whatwg/20120713#l-817>

[2] CSS Values and Units Module Level 3 (2015-12-08 20:47:23 +09:00 版) <https://drafts.csswg.org/css-values-3/#about-invalid>